Tissue growth involves cell proliferation, differentiation, and apoptosis to generate functionally organized multicellular patterns. Cell proliferation has to be regulated so as to maintain both the number of cells and their spatial organization. This regulation depends on interactions of cells with one another and with the extracellular matrix. The molecules which provide this regulation fall into several categories, including growth factors, oncogenes, tumor-suppressor genes, as well as extracellular matrix and cell adhesion molecules.
Growth factors were originally described as serum factors required to promote cell proliferation. Though growth factors are present in the circulation, most act as local mediators and originate from cells in the neighborhood of the responding cell. Growth factors bind to surface receptors on the responding cell and initiate an intracellular cascade, often involving activation of kinases and phosphatases. In addition to stimulating cell division, some growth factors, such as some members of the transforming growth factor beta (TGF-β) family, act on some cells to stimulate cell proliferation and act on other cells to inhibit it. Growth factors may also stimulate a cell at one concentration and inhibit the same cell at another concentration. Most growth factors also have a multitude of other actions besides the regulation of cell growth and division: they can control the proliferation, survival, differentiation, migration, or function of cells depending on the circumstance. For example, the tumor necrosis factor/nerve growth factor (TNF/NGF) family can activate or inhibit cell death, as well as regulate proliferation and differentiation. The cell response depends on the type of cell, its stage of differentiation and transformation status, which surface receptors are stimulated, and the types of stimuli acting on the cell. (Smith, A., et al. (1994) Cell 76:959-962; and Nocentini, G., et al. (1997) Proc. Natl. Acad. Sci. USA 94:6216-6221.)
Neighboring cells in a tissue compete for growth factors, and provided with “unlimited” quantities in a perfused system will grow to even higher cell densities before reaching density-dependent inhibition of cell division. Cells often demonstrate an anchorage dependence of cell division as well. This anchorage dependence may be associated with the formation of focal contacts, linking the cytoskeleton with the extracellular matrix (ECM). The expression of ECM components can be stimulated by growth factors. For example, TGF-β stimulates fibroblasts to produce a variety of ECM proteins, including fibronectin, collagen, and tenascin. (Pearson, C. A., et al. (1988) EMBO J. 7:2677-2981.) In fact, for some cell types specific ECM molecules, such as laminin or fibronectin, may act as growth factors. Tenascin-C and -R, expressed in developing and lesioned neural tissue, provide stimulatory/anti-adhesive or inhibitory properties, respectively, for axonal growth. (Faissner, A. (1997) Cell Tissue Res 290:331-341.)
Oncogenes (i.e. “cancer-causing genes”) are involved in reception and activation of growth factor signals. Mutations which hyperactivate oncogenes result in cell proliferation. Stimulation of a cell by growth factors activates two sets of gene products, the early-response genes and the delayed-response genes. Early-response gene products include myc, fos, and jun, all of which encode gene regulatory proteins. These regulatory proteins lead to the transcriptional activation of a second set of genes, the delayed-response genes, which include the cell-cycle regulators Cdk and cyclins. For example, the human T-cell leukemia virus type 1 (HTLV-1) Tax transactivator protein acts as an early response gene by enhancing the activity of a cellular transcription factor. The oncogenic properties of the Tax protein include transformation of primary T-lymphocytes and fibroblasts through cooperation with the a GTP-binding protein, Ras. Recently investigators have shown that Tax interacts with several PDZ-containing proteins. The PDZ domain, originally described in the Drosophila tumor suppressor protein Discs-Large, is common to membrane proteins thought to be involved in clustering receptors in growth factor signal transduction pathways. (Rousset, R., et al. (1998) Oncogene 16, 643-654.)
Tumor-suppressor genes are involved in inhibiting cell proliferation. Mutations which cause reduced or loss of function in tumor-suppressor genes result in cell proliferation. For example, the retinoblastoma gene product (RB), in a non-phosphorylated state, binds several early-response genes and suppresses their transcription, thus blocking cell division. Phosphorylation of RB causes it to dissociate from the genes, releasing the suppression, and allowing cell division to proceed.
Other gene products involved in cell proliferation, differentiation, and apoptosis are yet to be discovered. One method currently being utilized to help identify such new molecules involves comparison between quiescent and proliferative tissues. For example, a subtractive hybridization screen of human placental cytotrophoblast cells identified 20 genes whose expression levels rose due to EGF induction of cell proliferation. (Morrish, D. W., et al. (1996) Placenta 17:431-441.) Another method involves identification of molecules produced in cells treated with anti-tumorigenic agents, such as dithiolethiones. Presumably, the protective action of these anti-tumorigenic agents is associated with the induction of tumor suppressor gene products. (Primiano, T. et al. (1996) Carcinogenesis 17:2297-2303.)
The discovery of new molecules associated with cell proliferation and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cell proliferative and immune disorders.